Vocalizations Used in Play. Our previous studies of play behavior and the vocal activity accompanying it established that play is a robust and important behavior in development. One aspect was puzzling, however: squirrel monkeys were very noisy during play and this, coupled with the fact that play was intense and absorbing, would seem to expose playing youngsters (and perhaps the whole troop) to a greater risk of predation during play. Most animals are silent or nearly so when they play, and prior work in our lab had ruled out the possibility that this unusual noisiness was communicating anything of significance between the playing animals themselves. We used a group of four young monkeys housed separately but within earshot of a group of adults to test an alternative possibility that such calls act as signals to nearby adults instead. We found that adult females significantly increased their vigilance for predators during times when the youngsters emitted play vocalizations. This response was obtained whether or not the adults could see the youngsters. We conclude that one function of play vocalizations is to alert adults to increase their vigilance to protect themselves and/or the vulnerable young who are preoccupied in play.